mt_monashmedfandomcom-20200213-history
L0301P4 - Cellular Energetics and the Role of Enzymes as Biological Catalysts
__TOC__ Energy *the capacity to do work or change *many forms of of energy **gravitational **electrical **chemical **light **sound **thermal **mechanical ATP *Adenosine Triphosphate *the energy currency for cells *energy is stored in the bonds between the phosphate groups Potential Energy *chemical energy stored in chemical bonds Kinetic Energy *energy of movement Laws of Thermodynamics #energy cannot be created nor destroyed #*living cells obtain energy from the environment, e.g. food #energy is converted from not form to another, some may become unavailable to do work (unusable energy - entropy) #*transformations include breaking bonds, moving molecules across membranes Energy Conversions Total energy = useable + unusable H = G + T × S where: *H = total energy (enthalpy) *G = free energy (useable energy - what cells require for all chemical reactions) *S = entropy (unusable energy - mature of the disorder of the system) *T = absolute temperature Entropy *energy that becomes unusable *lots of energy is lost often in the form of heat and sound *if a chemical reaction increases entropy its products are more disordered or random than its reactants (breaking down) Changes in Free Energy *ΔG = ΔH - TΔS *ΔG reaction = ΔG products - ΔG reactants *if ΔG = -ve, the reaction is favourable **spontaneous, exergonic reactions release free energy *if ΔG = +ve, the reaction is unfavourable **non-spontaneous, endergonic reactions take up are energy *at equilibrium ΔG = 0 *ΔG gives no indication about the rate of reaction Transferring Energy in Cells *hydrolysis of ATP releases large amounts of free energy *after cleavage of the phosphates, they can be used to phosphorylate other molecules **e.g. glucose —> glucose-6-phosphate which allows it to cross membranes *reactions are often coupled to result in a net negative ΔG **like ΔH, the ultimate ΔG is just an addition of all the steps - the intermediate steps do not affect the final ΔG Exergonic Reaction * e.g. cellular respiration * also known as catabolism Endergonic Reaction *e.g active transport *cell movements *anabolism Enzymes *reactions in cells are inherently slow, thus catalysts are needed to increase the reaction rate (determined by the size of the activation energy barrier) *often the quaternary structure of the protein is what affects the function *does not alter ΔG *may change shape when they bind to the substrate How They Catalyse *lower energy barrier *offer an easier path *bring reactants closer together *place enzymes in the correct orientation *place physical strain on the substrate Catalysis *substrate binds to active site *held there by H bonds, electrical attraction or covalent bonds Factors that Affect Enzyme Function *substrate concentration **Michaelis Menten Kinetics: ***add substrate = rate of reaction increases but will reach a maximum rate where all the active sites have been taken up *enzyme concentration *temperature *pH *prosthetic groups **permanently bound to the enzyme **e.g. heme groups, FAD *inorganic cofactors **permanently bound **e.g.ions - iron, copper, zinc *coenzymes **not permanently bound - move from enzyme to enzyme **changed by the reaction and then released from the enzyme **e.g: NAD, ATP, Coenzyme A Regulation of Enzymes Inhibition *mostly reversible (competitive, uncompetitive and non-competitive inhibitors) *but there can also be irreversible ones Reversible Inhibition #Competitive #*blocks active site #Uncompetitive #*binds near active site blocking release of product #Non-competitive #*binds at an allosteric site preventing substrate from binding = no product produced Allosteric Regulation *binding of an effector molecule not at the active site *causing conformational change of the protein Inhibitor and Activator *allosteric enzymes are made up of multiple subunits - of which include the catalytic subunit and the regulatory subunit *inactive form **when the enzyme is inactive, it cannot accept substrate **binding of a allosteric inhibitor ensures that it stays in the inactive form **no product formation *active form **when the enzyme is in the active form, it can accept substrate **binding of an allosteric activator makes it more likely that the active form will occur **product will be produced Feedback Inhibition *inhibition via negative feedback *increased concentrations = inhibit function